Nonwoven fabrics used as filtration media have some required filtration-related properties i.e. that they are permeable to the fluid being filtered yet have high filtration efficiency. High permeability to the fluid being filtered is desirable, or even required, as less energy is needed to move the fluid through the filter media. High filtration efficiency is, of course, desirable and required as it allows the filtration media to more effectively remove contaminants in the fluid being filtered. Filtration properties can be quantified using tests such as Frazier Permeability, dP (pressure drop), PFE (particle filtration efficiency) and Index.
In many applications, filtration media are required which have structural integrity by themselves for conversion into various shapes. For example, the filtration media can be folded into a pleated shape that gives far more surface area than a non-pleated shape in the same space. However, often the filtration media is not stiff enough but requires a specific support structure, for instance, a mesh to maintain the desired shape of the filter.
Large fibers in a filtration media provide stiffness for pleating but undesirably degrade filtration efficiency. Further, some stiff filtration media are difficult to fold and may not “hold” the pleat, allowing the pleat to close and degrading filtration properties. Small fibers in a filtration media improve filtration efficiency and foldability but reduce stiffness.
As a matter separate from the stiffness issue, filters provided with a tacky layer are known. The purpose of such a layer is to better trap impurities from the flowing media.
An example of a tackified filter has been discussed in WO-A2-2007/084953, which discloses a tacky allergen trap and filter medium. The allergen trap includes a woven or nonwoven substrate having at least one layer. The trap is treated with a tacky adhesive the purpose of which is to trap the allergens, like dust mites. The WO document focuses mainly on providing cushioning material, like mattresses, pillows and furniture cushions with one or more allergen retaining layers. As a secondary product a filtration media provided with at least one layer bearing a tacky adhesive is discussed. The only teaching of the WO document relating to filtration media is that the tacky material is incorporated into a filtering element as a filtering medium whereby the only purpose of the tacky adhesive is to capture and hold microscopic dust-like particles. No other function of the tacky adhesive has been discussed or suggested.
A problem area, already mentioned briefly above, where specific stiffness of a filtration material is needed, is various wireless filters, i.e. filters that have no backing mesh or hole plate, but that are self-supporting. The present filter materials are subject to high expectations trying to fill the customers' needs, which sometimes require opposite measures to take. For instance air filters should have high particle separation efficiency, they should have minimal pressure drop, they should be light, they should be recyclable, they should be stiff enough to hold their pleats etc., and the filter is supposed to meet all these requirements over its entire lifetime in all of its more or less expectable working conditions. In wireless media filter manufacturing the media occasionally comes in contact with compression rolls and pleating surfaces within the production line. Additives to the media for tack enhancement can have a breakdown in the internal adhesion to the media and be deposited on the production line surfaces. With continued production the tack buildup can produce negative effects, i.e. either pull at the media deforming it or the buildup can be picked off the surfaces of the production equipment and be deposited back on to the. Certain additives are possible to be included which both reduce the tackiness and improve the filtration efficiency over the tackiness alone.